At first thought, an e-ink smartphone sounds like a terrible idea. Ugh, all that lag. But think about the light weight, low cost, and insane battery life, and you can see why eInk, the company behind the screen in Nooks and Kindles, is pushing its new prototype phone hard.

[We] were blown away by just how light it was, about how sharp text looked on its screen and how long it could last... Perhaps because it can use a smaller battery or because it has an older ARM processor - Charbonnier said it has a Qualcomm A5 chip - the reference design weighed a mere 80 grams and felt as light as paper in our hand... The device is designed to last at least a week on a charge and cost cost as little as 150 euros unsubsidized.

Sounds good, huh? Running an aged version of Android—2.3!—it had a handful of apps, but that at least shows that an e-ink phone is possible. Of course, it would be a world of compromise; while battery life is a major draw, that's not much use if the thing is riddled with so much lag that you want to stamp its little screen to smithereens:

Navigating between menus was painfully slow, and though the device supports multitouch gestures such as pinch-to-zoom, it wasn't always responsive. There was also significant lag when redrawing the screen, which is always a problem on E Ink screens but was particularly sluggish on this device.

But, hey, this is a proof-of-concept, not a finished product. eInk's alternative offering is along the lines of that Yota dual-screen phone: it's contemplating offering replacement e-ink back covers for phones that could cost somewhere around $50. That's a neat idea, and swapping a second screen off and on to your phone as needs dictated actually sounds quite sensible. Would you be interested? [Laptop]